1. Field of the Invention
The invention relates generally to implantable medical devices and more particularly to shielded feedthrough structures that connect one or more implantable patient leads to various operational circuitry within the housing of the implantable medical devices while maintaining a hermetic seal of the devices.
2. Description of the Related Art
A variety of implantable medical devices have been developed and employed for long-term implanted monitoring of one or more patient physiological conditions and/or delivery of indicated therapy. Implantable cardiac stimulation devices are one particular category of implantable devices which are adapted to monitor the patients' physiological conditions, including their cardiac activity, and to generate and deliver indicated therapy to treat one or more arrhythmic conditions. Implantable cardiac stimulation devices typically include either a high voltage circuit for generating high voltage waveforms, a low voltage circuit for generating relatively low voltage pacing stimuli, or both low voltage and high voltage circuits that generate waveforms for delivery to patient tissue. These devices also typically include a microprocessor-based controller which regulates the delivery of the high voltage or pacing pulse waveforms. The high and/or low voltage circuits and the controller circuitry are generally encased within a biocompatible can or housing along with a battery to power the device.
Implantable cardiac stimulation devices typically also include one or more implantable patient leads with associated electrodes. The implantable patient leads are typically connected at one end to a corresponding electrode that delivers therapy to the patient's heart and at the other end to the high and/or low voltage circuitry and controller in the can or housing. Because of the highly corrosive liquid implanted environment and because the materials and operations of the electrical circuitry are not compatible unless properly isolated from each other, the connection between the leads and the circuitry inside the housing must be such that a hermetic seal is maintained. Thus, typically, connections between the electrical circuits disposed inside the housing of the implantable device and the patient leads outside of the housing are achieved through one or more feedthrough assemblies.
The feedthrough assemblies provide for connection between the leads outside of the housing and the circuitry inside the housing while maintaining a hermetic seal. Additionally, the feedthrough assembly of an implantable medical device generally includes circuitry for filtering the electrical signals received through the patient leads so as to attenuate the spectrum of unwanted frequencies before they reach the circuitry inside the housing of the implantable device. Prior art implantable cardiac stimulation devices generally achieve this filtering through multilayer ceramic type capacitors, such as discoidal capacitors. These discoidal type capacitors are typically disposed inside the feedthrough assembly. The capacitors are very specialized, difficult to manufacture, and are therefore expensive. Because of the specialized type capacitors, prior art feedthrough assemblies occupy premium space.
As implantable medical devices are configured to be implanted inside a patient's body, their overall dimension cannot exceed certain predetermined sizes. An increase in the size of the implantable device may result in added discomfort to the patient while a decrease in size can reduce potential irritation for the patient. Further, due to the limited possible size of these devices, the amount of space inside the device is also limited. Thus, the size of various components used in an implantable medical device is an important design consideration. Smaller components may create space for additional features, while a larger component may limit the size for other features and components. The large size of the feedthrough device due to the inclusion of the filtering capacitors thus reduces the amount of space within the housing that can be used for circuitry or therapy delivering components. Hence, there is a need for a feedthrough structure that provides filtering capability but has a reduced footprint within the housing to thereby allow for more space for other components.